1. Technical Field
The present invention relates generally to circuit design and evaluation software, and more particularly, to a methodology and computer program that uses circuit moments to compute currents in a metal layer of an integrated circuit.
2. Description of the Related Art
Metal layer currents or “wire currents” in an integrated circuit must typically be evaluated for each design for a number of reasons. The metal layer interconnects must be properly sized to handle both peak current levels and power dissipation due to ohmic losses. Further, as design technology is scaled to decrease the circuit area, current densities in the interconnects increase, dramatically increasing problems due to electromigration. Electromigration causes movement of metal ions in the metal layer, deforming the conductors over time and potentially causing circuit failure. Voids formed in the conductors cause increased resistance and open circuits, and in particular, the reduced cross-section further increases current density and ohmic heating. Migration of the conductors toward other conductors can cause short circuits between conductors, and consequent failure of the integrated circuit. Both the peak and average values of current through a conductor are important factors in analysis of electromigration effects, and present reliability models typically require computation of the peak, root-mean-square (rms) and average current values for each conductor.
Complete determination of all currents in all interconnects in an integrated circuit is possible by simulating the circuit using a simulation program such as SPICE, but is time-consuming and requires an extensive amount of computing resources. Other techniques approximate the currents from timing models as triangular waveforms at each transient switching event and/or attempt to prune the set of interconnects for which currents are calculated, by selecting which interconnects are most likely to fail. However, for an analysis of potential electromigration failures and reliability values (mean time to failure), a full simulation is typically required. The timing model analyses will typically under or over-predict peak current values, depending on whether a conservative model is used. Pruning of the set of interconnects may miss conductors that have a high current stress vs. size and/or spacing.
It is therefore desirable to provide a method and system for accurately determining peak, average and rms current levels for all interconnects in metal layers of an integrated circuit without excessive computational burden.